Image capturing devices are commonly used to capture scenes, persons, events, settings, etc. Image capturing devices may come in a variety of types including, for example, digital still cameras and video cameras. A typical image capturing device includes a lens and an electronic image sensor, such as a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) array. The electronic image sensor comprises a two-dimensional array of light sensitive pixel elements that produce electronic signals in response to light. The electronic image sensor therefore outputs electronic signals that are related to the amount of light impinging on the individual pixel elements of the array.
One problem encountered with an electronic image sensor is noise that is picked up during an image capturing process. In addition to creating an output signal based on light impinging on the sensor elements, the sensor array may also create output signals based on noise, where the noise is generated by unwanted light or generated due to thermal effects or stray currents in the electronic image sensor. This noise corrupts a captured image and therefore is highly undesirable.
A first prior art noise reduction approach involves dark frame subtraction, wherein the prior art camera captures a regular image and then captures a dark image. The dark image is captured at about the same exposure time and generally is captured when the shutter is closed, i.e., when the electronic image sensor is dark. The prior art approach thereafter subtracts the dark image information from the regular image information in order to remove the noise from the regular image. This improves the overall image captured by the image capturing device.
However, this prior art approach has several drawbacks. Capturing a dark image after a regular image requires an exposure for each image capture (i.e., a double exposure). This increases shot-to-shot time, and moreover increases processing time. This processing time requires a time component for capturing the dark image and a time component for subtracting the dark image.
A second prior art noise reduction approach employs a fixed dark cell in the camera. This dark cell may be a pixel element that does not participate in an image capture, but instead is a pixel element that is sealed off or covered and cannot generate anything but a dark reading (i.e., a blind cell). The dark cell gives an average or approximate noise reading for the entire electronic image sensor and does not give a reading for each individual pixel element. This average noise value is then subtracted from all pixels of the captured image. This prior art approach is much faster than a full dark image capture, and does not require a dark image capture after the regular image capture, i.e., the dark cell may obtain a reading concurrently with the image capture.
However, there are drawbacks in the second prior art approach. The dark cell is less effective at noise reduction. The dark cell output may not accurately reflect the varying noise levels in each individual pixel element. The dark cell approach therefore does not take into account variations in pixel elements and moreover becomes less accurate over long exposure periods where pixel-to-pixel noise variations can be significant.
Therefore, there remains a need in the art for improvements in noise reduction for electronic image sensors.